1. Technical Field
The present invention relates to a partial heat treatment method in a salt bath at low temperature, and more particularly, to a partial heat treatment method in a salt bath at low temperature, wherein heat treatment which enables a specific metal to penetrate to the surface of a workpiece to thus increase surface hardness is performed by an immersion process in a molten salt solution at low temperature, thereby preventing corrosion resistance from deteriorating.
2. Description of the Related Art
Heat treatment is widely utilized to enhance surface hardness of a workpiece. Typically, heat treatment is performed by bringing a workpiece into contact with a gas or molten salt solution for carburizing or nitriding at high temperature to thus diffuse the carbon or nitrogen atoms to the surface of the workpiece.
Heat treatment using a salt may be used to increase hardness of a metal having high corrosion resistance such as iron (stainless steel) containing chromium. An increase in hardness is achieved via formation of a nitride precipitate or a carbide precipitate abbreviated to “nitride” or “carbide”.
As such, a nitride or carbide is configured such that chromium is precipitated with respect to nitrogen or carbon. When a non-uniform surface structure is formed through precipitation in this way, a difference in electronegativity may occur between a portion where chromium is lacking due to removal of chromium and a portion where chromium is precipitated. Such a difference allows for action as a kind of galvanic cell, and thus a metal product may easily corrode. The precipitation easily takes place when the same element as in the precipitate is present in a large amount in the workpiece or it is easy to permeate particles due to high heat treatment temperature.
Although a workpiece needs to be totally hardened, a part such as a ferrule is required to have high hardness only on a portion thereof.
FIGS. 1A and 1B are a cross-sectional view and a perspective view, respectively, illustrating a ferrule having a predetermined shape used to connect two pipes.
FIG. 1A is a cross-sectional view illustrating the connection structure of two pipes using a ferrule.
When the two pipes, for example, a front pipe 11 and a rear pipe 15 are connected, the ferrule functions to close a gap between the front pipe 11 and the rear pipe 15 and is responsible for swaging the pipes to ensure a sealing function and for preventing separation of the pipes.
The ferrule may include a front ferrule 13 and a back ferrule 14.
The back ferrule 14 plays a role in that while the rear (tail portion 14b) of the back ferrule 14 is pushed by a nut 12 for tightening the pipes, a force is transferred to the front ferrule 13. As such, while the nut 12 rotates, it tightens the pipes, and thus rotational torque is created. The back ferrule 14 performs a linear motion in the travel direction of the nut 12 when the nut 12 is moved forward while rotating.
By the back ferrule 14, the lower beveled portion of the rear of the front ferrule 13 is lifted up, and a nose portion 14a swages the pipes to thereby prevent the separation of the pipes.
Accordingly, desired purposes may be achieved only when the hardness of the nose portion of the back ferrule 14 is high. If the entire back ferrule 14 has high hardness, rotational torque of the nut 12 is not efficiently absorbed, thus increasing brittleness. Thus, only the nose portion 14a of the back ferrule 14 has to be selectively hardened.
FIG. 1B is a perspective view illustrating the ferrule. The ferrule is ring-shaped and the nose portion thereof is transformed by a force applied to the front while being twisted from the back. Hence, the nose portion needs to be particularly hardened.
The front ferrule 13 includes a support portion 13b to which pressure is applied while the nut 12 is tightened, and a nose portion 13a which receives the applied pressure and thus undergoes irreversible transformation and closes and swages the edge of the pipe. As high friction and force are applied in the course of transformation, the nose portion 13a has to possess high hardness and elasticity. The parts that selectively require high hardness on a predetermined portion, such as the ferrules 13, 14, should undergo selective partial hardening treatment.
When high hardness is required only on a predetermined portion in this way, such a portion is hardened through partial heat treatment. Typically, a partial heat treatment method includes plating a workpiece with a different kind of metal, wherein the resulting plating is used as a mask against heat treatment. Specifically, the workpiece is plated with a different kind of metal, and the plating is removed from a portion to be hardened, so that the surface of the workpiece is externally exposed. Then, heat treatment is performed, and thereby the portion which is not externally exposed blocks permeation of nitrogen or carbon due to the plating, and thus precipitation does not easily occur. Consequently, only the exposed portion is selectively hardened.
However, upon long-term heat treatment at high temperature while carrying out partial hardening through such a plating process, chromium is excessively precipitated on the portion which undergoes partial heat treatment, undesirably deteriorating corrosion resistance.